1. Field of the Invention
The present invention relates to an ignition timing control apparatus for an internal combustion engine, and more particularly to an ignition timing control apparatus for an internal combustion engine having a plurality of cylinders, wherein operation of the engine is switchable between partial-cylinder operation, which operates some of the cylinders, and all-cylinder operation, which operates all of the cylinders.
2. Description of the Related Art
An exemplary ignition timing control apparatus that determines whether knocking has occurred in an internal combustion engine based on an output signal from a knock sensor and corrects the ignition timing according to a result of the knocking determination is disclosed in Japanese Patent Publication No. 2844418. According to the known ignition timing control apparatus, the octane of the fuel being used is estimated, and a learning value of the estimated octane is calculated according to a result of the knocking determination. Further, the estimated octane is initialized or updated, and the ignition timing is corrected according to the estimated octane.
In the conventional ignition timing control apparatus, partial-cylinder operation of an internal combustion engine is not taken into account. Therefore, if the conventional ignition timing control apparatus is directly applied to control the ignition timing of the engine that is switchable between partial-cylinder operation and all-cylinder operation, then several problems arise.
When the combustion condition in the operating cylinders in partial-cylinder operation is good, the learning value is updated in such a direction that the ignition timing is advanced. When the operation of the engine changes from partial-cylinder operation to all-cylinder operation and the combustion condition in those cylinders which have started to operate from the halted state, is not good, knocking is likely to occur if the learning value updated in partial-cylinder operation is applied.
It is an aspect of the present invention to provide an ignition timing control apparatus which appropriately controls the ignition timing of an internal combustion engine that is switchable between partial-cylinder operation and all-cylinder operation according to the result of knocking determination, thereby obtaining a maximum engine output within the range where knocking is suppressed.
In order to attain the above aspect, the present invention provides an ignition timing control apparatus for an internal combustion engine having a plurality of cylinders. The operation of the engine is switchable between partial-cylinder operation, which operates some of the cylinders, and all-cylinder operation, which operates all of the cylinders. The ignition timing control apparatus includes basic ignition timing calculating means, a knock sensor mounted on the engine, knocking determining means, correcting means, and learning value calculating means. The basic ignition timing calculating means calculates a basic ignition timing according to an operating condition of the engine. The knocking determining means determines whether knocking has occurred based on an output of the knock sensor. The correcting means calculates a correction amount according to a determined result from the knocking determining means to suppress knocking, and corrects the basic ignition timing with the calculated correction amount. The learning value calculating means calculates learning values of the correction amount calculated by the correcting means. Further, the learning value calculating means calculates the learning values corresponding, respectively, to partial-cylinder operation and all-cylinder operation, and the correcting means uses one of the learning values calculated by the learning value calculating means according to the operating condition of the engine.
Accordingly, the correction amount of the ignition timing is calculated according to a result of the knocking determination to suppress knocking, and the learning value of the correction amount is further calculated. The learning value is calculated corresponding, respectively, to partial-cylinder operation and all-cylinder operation. The calculated learning value is applied as the correction amount according to the engine operating condition. Therefore, in the transition state from partial-cylinder operation to all-cylinder operation or vice versa, a learning value suitable for the respective operating condition is applied to thereby obtain a maximum engine output within the range where knocking is suppressed.
Preferably, in all-cylinder operation, the learning value calculating means calculates a learning value to be applied in an operating condition where an exhaust gas recirculation is performed, when the exhaust gas recirculation is being performed, and calculates a learning value to be applied in an operating condition where the exhaust gas recirculation is not performed, when the exhaust gas recirculation is not being performed.
Preferably, the basic ignition timing calculating means calculates the basic ignition timing according to a number of operating cylinders of the engine.
Preferably, the correcting means uses one of the learning values as the correction amount when the learning value is updated by the learning value calculating means so that the ignition timing is advanced.
Preferably, the correcting means includes limit value calculating means to calculate a limit value of the correction amount using the learning values corresponding, respectively, to partial-cylinder operation and all-cylinder operation, and limiting means to limit the correction amount using the limit value calculated by the limit value calculating means.
Preferably, the knocking determining means includes noise level calculating means to calculate a noise level based on the output of the knock sensor, gain coefficient calculating means to calculate a gain coefficient according to a number of operating cylinders of the engine, and determination threshold calculating means to calculate a determination threshold by multiplying the noise level by the gain coefficient, wherein the knocking determining means determines that knocking has occurred when a maximum value of the output of the knocking sensor exceeds the determination threshold.